The instruments include the Mercury Dual Imaging System (MDIS), the Gamma-Ray and Neutron Spectrometer (GRNS), the X-Ray Spectrometer (XRS), the Magnetometer (MAG), the Mercury Laser Altimeter (MLA), the Mercury Atmospheric and Surface Composition Spectrometer (MASCS), and the Energetic Particle and Plasma Spectrometer (EPPS). The instruments communicate to the spacecraft through fully redundant Data Processing Units (DPUs). The instruments are labeled in the picture below.
The process of selecting the scientific instrumentation for a mission is typically a balance between answering as many science questions as possible and fitting within the available mission resources for mass, power, mechanical accommodation, schedule, and cost. In the case of MESSENGER, the mass and mechanical accommodation issues were very significant constraints. Payload mass was limited to 50 kilograms (110 pounds) because of the propellant mass needed for orbit insertion. The instrument mechanical accommodation was difficult because of the unique thermal constraints faced during the mission; instruments had to be mounted where Mercury would be in view but the Sun would not, and they had to be maintained within an acceptable temperature range in a very harsh environment. In each case the mass includes mounting hardware and thermal control components, and the power is the nominal average power consumption per orbit; actual values vary with instrument operational mode.